And I'm Robert Siegel. We're going back in time now, millions of years or possibly tens of millions of years. We're talking about the age of the Grand Canyon and a new research paper that's generating a lot of impassioned debate.

BLOCK: Rebecca Flowers(ph), a geologist from the University of Colorado, says her research shows that that geological marvel is much older than previously thought. Prevailing scientific wisdom says the Grand Canyon is about six million years old, but Flowers puts the age at more like 70 million years. Joel Achenbach has written about this furious debate in the Washington Post. Hey, Joel.

JOEL ACHENBACH: Hey, Melissa.

BLOCK: And let's give a sense of the tone of this debate here, because another key Grand Canyon geologist called Flowers' conclusion ludicrous. He says it's out in left field. Why is this such a hot topic?

ACHENBACH: Yeah, it's interesting that Rebecca Flowers, Dr. Flowers, actually collected a lot of her samples with this other scientist, Carl Carlstrom(ph), who is a critic of her conclusions because, you know, they have to raft down the river and chip away at the rock on the canyon walls and get these samples.

But this is a basic question of: What are we looking at here? When you stand on the rim of the Grand Canyon, what are you looking at? Now, obviously it's a canyon carved by a river. You see the Colorado River at the bottom. So the causality seems pretty clear, except this new hypothesis says that much of the canyon goes back 70 million years and was carved by a different river, in fact two different rivers, neither of which was the Colorado River.

And so this is - this is a new idea based on some new scientific techniques, and it has really roiled the waters of that community of geologists.

SIEGEL: And you're saying they were both on the same boat, rafting down the Colorado River, taking samples at the same time. How did it end up that these two geologists have become so diametrically opposed and that this is so contentious?

ACHENBACH: Well, this is science. I mean, this is a pretty big question: How old is the Grand Canyon? You have a veteran geologist who spent years figuring this question out, and there is a lot of evidence for a Grand Canyon that's about six million years old, or maybe a little bit less.

I think what the new hypothesis says is that a lot of this canyon was abandoned for a while, it was dry, so you had this sort of canyon just sitting there. And the Colorado opportunistically came along at some point prior to about six million years ago and occupied some of these ancient channels.

So I think that what we're seeing here is actually science at its best. And, of course, it's going to cause some contentiousness, and the people who've developed the old theory are going to say hang on, you know, you young whippersnapper, you haven't proved this idea.

(LAUGHTER)

BLOCK: Yeah.

ACHENBACH: And next week they'll have a chance, these scientists, to present their findings at the annual fall meeting of the American Geophysical Union in San Francisco. And they'll go back to back to back, and they will give very different interpretations of what happened there. But this is how science works.

I mean, keep in mind they're in unknown terrain. I mean, they're pushing the edge of what's knowable, and they're coming up with new techniques that are quite remarkable. I mean, just the idea of it, that they can look at some tiny little crystal in an ancient rock and figure out what the landscape looked like, to me is a fascinating achievement.

BLOCK: What do you figure the tone of that meeting will be when these competing theories are put back to back, as you say? Are sparks going to fly or is this a pretty congenial debate?

ACHENBACH: Well, you know, these are geologists. They're scientists. I don't know that sparks fly. I think it'll be testy. I think it'll be some forceful arguments, and I don't think they're going to have a big Cumbaya moment, but I think they respect each other's intelligence, and they believe in what they're doing here, trying to decode the landscape, decode the mystery of the Grand Canyon.

BLOCK: That's Joel Achenbach, science writer with the Washington Post. And now to somebody who probably thinks about this question every day. Carl Bowman has been a park ranger at Grand Canyon National Park for 25 years. Mr. Bowman, welcome to the program.

CARL BOWMAN: Good to talk with you.

BLOCK: And do you get this question a lot from visitors from there, how old is the Grand Canyon?

BOWMAN: It's a very common question. Here's this fabulous scene out there and it takes a while to just drink it all in, but that's one of the things that comes to their mind is how did it form, and how long has it been here.

BLOCK: Well, what do you tell people when they ask how long it's been there?

BOWMAN: We tell them that it's probably less than six million years old, but geologists are still studying the problem. There's still a lot to be worked out on the history of the Grand Canyon.

BLOCK: Does it change anything for you if you start thinking, gosh, maybe it is 70 million years old, not six million but 70 million years - does it change anything about how you see where you work?

BOWMAN: Actually it doesn't a whole lot to me, and the reason is that when we look at the canyon, we've got all those spectacular layers of rock that are piled up out there, and we know that those layers were lifted up towards the end of the age of the dinosaurs, somewhere around 70, 75 million years ago they started rising up. And most geologists, not all of them, but most geologists think that the Grand Canyon is less than six million years old.

Well, that leaves a long gap in time there, and something had to be going on. And so I hear news like this, and I think, oh, another piece of the puzzle as to how this landscape evolved from a beach that the dinosaurs were walking on 70 million years ago to this, you know, spectacular canyon a mile and a half above sea level today.

BLOCK: What's your favorite time to be in the Grand Canyon looking around?

BOWMAN: Probably in the evening in October, when the heat's not so blistering down there inside of the canyon, and you get those cool breezes and the fall colors, and generally that time of year, the air's nice and clear. So it's just spectacular.

BLOCK: I suppose there's never a bad time to be in the Grand Canyon.

BOWMAN: There really isn't. When I'm working out on the rim, and we get fog banks and cloud banks, especially in the wintertime, that just obscure the canyon, you walk up to the Grand Canyon, and you might as well be walking up to a bed sheet hanging on a clothes line, can't see a thing over the rim.

(LAUGHTER)

BOWMAN: But it doesn't last for long, and pretty soon the clouds start to break up, and just be patient.

BLOCK: It'll change.

BOWMAN: Yeah, it sure will.

BLOCK: Well, Carl Bowman, it's great to talk to you. Thank you so much.

BOWMAN: Sure, thank you, Melissa.

BLOCK: That's Park Ranger Carl Bowman, speaking with us from the headquarters building on the south rim of Grand Canyon National Park. Transcript provided by NPR, Copyright National Public Radio.